Michael Bloomberg has offered to pay $15 million to the U.N. in the wake of the U.S. withdrawing from the Paris Agreement. (Christophe Petit Tesson/European Pressphoto Agency)

When President Donald Trump announced that he would withdraw from the Paris Agreement last week, he took with him a large chunk of the money pledged to the United Nations to pay for the fight against climate change.

Former New York City mayor Michael Bloomberg caused a ripple of optimism by promising to provide up “up to $15 million” to cover the U.S. contribution.

But $15 million doesn’t even begin to fill the gap in the U.N.’s Green Climate Fund left by the U.S. withdrawal from the Paris agreement.

The Green Climate Fund is the main vehicle to help poor countries fight climate change. Under President Obama, the U.S. pledged $3 billion to this climate bank. While $1 billion has already been handed over, the remaining $2 billion will no longer be transferred.

This leaves a huge gap in the funds designated to the bank. Removing $2 billion from the pot reduces available funds by around a fifth, from $10.3 billion down to $8.3 billion.

Over 40 national governments have so far pledged to the fund, as well as three regions in Belgium and the city of Paris.

While the U.S. was only the 11th most generous country in terms of money pledged on a per-person basis, its absolute contribution was twice as large as the next biggest contribution, $1.5 billion from Japan.

Bloomberg’s $15 million contribution will not go toward the Green Climate Fund but toward the running costs of the United Nations’ climate agency’s secretariat.

This body undertakes tasks such as preparing the groundwork for the implementation of the Paris Agreement commitments. The U.S. has typically contributed up to $4.5 million a year to this fund.

The Green Climate Fund, on the other hand, is designed to fund projects on the ground that will directly reduce emissions and help citizens adapt to the impacts of climate change. Projects approved so far include coastal adaptation in Tuvalu, water conservation in Morocco, and scaling up hydro power in Tajikistan.

While the U.S. has said it wants to leave the Paris agreement, the goals within the deal remain the same — including on climate finance.

By signing and ratifying the deal, developed nations effectively promised to provide $100 billion in climate finance every year by 2020, and agreed to set an even higher target in 2025. Much of this is expected to be channeled through the Green Climate Fund.

Without the financial power of the U.S., the world’s largest economy, this goal starts looking harder to meet, and has left other countries deliberating whether they should cover the gap.

Germany has signaled it would not. “No, this would send a wrong signal,” says Barbara Hendricks, Germany’s environment minister. “No country that refuses joint international solidarity can expect that others just step in to fill the gap. By the way, developing countries show respect and sympathy for this position.” Germany has so far contributed around $1 billion to the fund.

Other governments echoed Germany’s position, with a Czech official telling The Washington Post that there were no plans to increase funding to cover the gap left by the U.S., and an Australian official adding that the government would simply honor its existing commitment.

Others were more open to the idea. “When it comes to their financial contributions to the Green Climate Fund and other institutions, it should really encourage other developed nations to provide more in the absence of American leadership,” said Erik Solheim, head of the United Nations Environment Program.

And Sweden, the largest contributor to the Green Climate Fund on a per-person basis, has not ruled out the possibility of increasing their donation.

“It is these countries that pay the highest price of climate change that we, the developed countries — not least the USA — has caused. Climate financing is a question of justice and taking responsibility for our actions.”

She added that she would discuss the matter with colleagues in Europe. Francesco La Camera, director general for sustainable development, energy and climate at the Italian environment ministry, also said that he would discuss the issue of raising funds at a meeting of European Union environment ministers in a couple of weeks’ time.

Nations will not be able to hide from the issue for long. Once the Green Climate Fund has spent 60 percent of its funds, it begins a formal process of replenishing its pot of money, and countries will have to decide how much they want to allocate in this second round.

The Green Climate Fund has so far approved $2.2 billion of projects. With the size of the pot shrinking from around $10.3 billion to just $8.3 billion, some experts expect this 60 percent trigger could be hit as early as 2018.

While there are no formal targets, countries should ensure that the next round provides at least $10 billion in funds to ensure that there is an “upward trajectory,” says Brandon Wu, director of policy at ActionAid USA, “so countries will have to figure out how to meet that kind of number without the U.S.”